shooting a unit that is further away doesn't seem to have any effect, only characters <10wounds can't be shot. also i think there was a clarification/some mention in a live stream somewhere that even if the character is behind a unit that is is combat it still can't be shot, (not sure about that one though if anyone could confirm this please do so)...

"Q: If a model cannot shoot at the closest visible enemy
unit for some reason (e.g. it is within 1" of one of
your units) but the next closest visible enemy unit is a
Character, can that model then target the character?
A: No."

I think GW cocked up on the Characters, being further away does not protect them because your enemy can move and to be of value they have to close with the enemy. So all you do if you want to shoot a Character is to flank the blocking unit some how, which will be easier the closer the character gets.

Example 1: My Spiritseer needs to be within 6" to "spirit mark" an enemy unit. Sure he can have a wraith unit in front of him. But to get that close means he is exposed for one enemy turn. So say he is attacking SM. They use a unit of bikers to flank the wraith unit and close with the Spiritseer. They can then legitimately shoot him and have now enveloped the wraith unit ta-boot.

Example 2: SM are attacking my lines and a character is in support of the assaulting unit. I fly around the SM unit with a Falcon and make the character the closest target. Two heavy weapons and an assault weapon then pump multiple high strength shots into the character and he has no ablative troops to absorb the grief. And the Falcon no longer has to worry about facing armour so it dose not matter if its rear armour is facing the remaining enemy troops.

Example 3: Deep strike, nuff said.

The other key point is that the unit that carries out the flank attack is not the original target of the Character and his troops. They would be to the right or left of that that unit so that the movement gives them the best chance of moving into a firing position. Though skimmers can still fly over one unit to get to another.

I just found out that my Rangers can target any character they choose regardless of the proximity of any other unit, unless they are in CC of course. But if my snipers can it probably means most if not all can and something like the Callidus sniper can, your characters are toast.

I also found that you can shoot your pistols in the shooting phase even if you are locked in combat, though in CC the only target are the ones you are fighting.